We have been using OpenERP Bitnami image on Google Cloud for over a year now. Lately we have been receiving notifications that this image will be unsupported in future and existing instances will be deleted by June.

What are our options? How can we migrate to another service such as AWS?

We do not know anything about deleting instances in GCE. OpenERP will no longer be supported to make way for Odoo. If you have received this message from Google I propose two options:

Upgrade to Odoo stack:I tried this process to upgrade Odoo 8 to Odoo 9 and it works fine.

Migrate to the other cloud:In your actual instance follow this section to create a backup. Launch other instance with other bitnami application (no matter which is) in the new cloud and follow this guide to restore your previous backup.

Yes, if you choose the second option, you can install your current OpenERP in other instance, but you are not only migrating the data, you are overwriting the new application (that's why I say that it does not matter the application) and replacing it with your ERPNext (including data, of course).